I'm not averse to the concept of others improving my answers. However an answer I provided to a question has been edited and the edit's don't sit well for me because they aren't really what I would consider as solutions:

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That does look like a bit of a different answer, which means it's not appropriate being added to yours (except by you, or if it's Community Wiki). I would remove that part, and respond to the OP suggesting they post it as an answer. They may not have understood that they can answer their own question.

Let them know they may even get some votes for it, too.

In general, I'm open to anyone editing my question or answer, but as soon as it's not what I meant, or especially something I disagree with, I tend to think it's more appropriate that they post their own answer.

This edit does seem over the top, I would have made the additions a comment on your answer (maybe even a separate answer, I don't know the subject matter so I can't tell how important they are). I don't think it's unacceptable behavior, unless he makes a habit of doing that.

A proper response would be to revert the edit, and leave a comment on your answer telling the editor you didn't like his changes and suggest reposting them under his name. Note that @name in a comment on a post works to notify anyone who edited the post.

I have a one-revert rule: it's ok to (occasionally) make an edit the original author doesn't like, and it's ok for the author to revert (except edits for reason of moderation, e.g. to remove insults). If the editor insists and won't let you have your post the way you want it, you can get a moderator involved; a moderator can lock your answer to prevent edits (even by you), and if the user is significantly disruptive, a moderator can write him a stern letter or suspend him.

Firstly, the fact that it was the OP of the question who modified your answer is irrelevant =)

If you don't believe that edits to your answer have improved your answer so it provides more value to someone asking the question that the OP asked, revert the changes. If they did add value, consider leaving them as is. If you feel that parts might make more sense being part of the question then move them to the question.

In this specific example, I'm not sure why the OP decided to add to your answer and it would make sense to move parts of what they added to the original question.